Vacuum switching devices are utilised in most modern medium voltage electrical installations. Vacuum switching devices are typically employed as part of a switchgear which is a broad term for the combination of electrical components used to control, protect and isolate electrical equipment and circuits. Switchgear generally comprise a switching device, such as a vacuum interrupter, an actuator for exerting and applying a force to switch the switching device and a detection system for detecting a switching requirement (including faults) in the electrical equipment/circuit.
Vacuum switching devices, commonly called vacuum interrupters, are well established as highly suited as the switching device in switchgear. A known vacuum interrupter is shown in FIG. 1. A vacuum interrupter of the type shown in FIG. 1 typically comprises an evacuated envelope or housing 10 formed by an insulating component 12 and metal end plates 14, 16. The housing 10 encloses a fixed electrode 20 and a moveable electrode 22 that are designed to engage and disengage mechanically to perform a switching function. Normally this movement is permitted without breaking the seal of the evacuated envelope 10 by means of a bellows or diaphragm arrangement 24. Typically each electrode comprises a contact assembly or contact 26, 28 coupled to a conducting rod which is called a rod or stem 30, 32.
A problem with existing vacuum interrupters is that the bellows or diaphragm arrangement is a weak point within the device. As the bellows both provide for the movement of the stem, and therefore the movement of the movable electrode/contact, and are part of the housing, after multiple actuations the bellows can wear out and fail. Typically, this failure leads to loss of vacuum within the housing. Due to the relatively large voltages employed, typically 1000V-50 kV, loss of vacuum in this manner causes a loss of insulation effect of the vacuum interrupter due to the Paschen's law. This causes the vacuum interrupter to fail to interrupt at the required low current. The success of vacuum interrupters has also led to many of the devices being in use for decades, much longer than their original intended usage, resulting in a higher risk of such mechanical failure than originally accounted for.
Vacuum interrupters and similar functioning devices are the key components within electrical switchgear, which may form or be part of a circuit breaker or motor control centre or other switching device. In present designs of switchgear an actuator is connected mechanically to the moving electrode (typically via the connecting rod or stem) of the vacuum switching device and acts to engage or disengage the moving electrode with the fixed electrode by acting on the stem. Conventionally, multiple vacuum interrupters are required for an electrical installation which often is a three phase circuit with one or more vacuum interrupters per phase, and a single actuator can then be used to actuate multiple vacuum interrupters. Consequently, the actuators used tend to be large and require additional components or multiple connections to each stem. Actuators may be of several types including magnetic, spring, hydraulic or pneumatic.
In the literature smaller actuators located within an evacuated chamber are described and may be used in some one-use switching or breaker devices. However, such devices are either direct current devices and/or low voltage devices and are unsuited to alternating current and/or medium voltage regimes due to unpredictable or unreliable switching behaviour under such conditions. Smaller actuators typically described in such breakers include Thomson coil actuators. However, such actuators are not of practical use in alternating current vacuum switching devices and their associated switchgear because the force generated for actuation relies on the inducement of eddy currents within conducting discs, which then repel and move an associated contact. However, the force required is too low for actuators used in alternating current and medium voltage regime switching devices. Furthermore, the eddy current is produced by changes in the magnetic field of the coil current, so the force only sustains while the coil current is changing. If the current changes by increasing, it soon gets too large to be provided by the supply, and if it changes by decreasing, it soon reaches zero. Thus the force is time limited. By contrast in a conventional magnetic actuator the force profile over time can be tailored to requirements by shaping a pulse of coil current, and can be continued indefinitely if required. Such smaller actuators, such as Thomson coil actuators also do not allow latching of a switch in an open or closed position because it requires a constantly changing magnetic field, reinforcing their intended use in breakers and single use devices. Finally, when large short circuit current is to be interrupted, this condition will induce large eddy currents which will interfere with the operation of the Thomson coil. This could result in uncommanded operation of the switch or prevent a commanded operation with potential catastrophic effects if used in a switching device for medium voltage. Such uncommanded operations are specifically forbidden in International standards concerning switchgear.
In summary, for at least the reasons outlined above, an improved vacuum switching device is desired.